The invention relates to a molecular electronics arrangement and a method for fabricating a molecular electronics arrangement.
Conventional silicon microelectronics will reach its limits as miniaturization advances further. In particular the development of increasingly smaller and more densely arranged transistors of hundreds of millions of transistors per chip will be subject to fundamental physical problems in the next ten years. If structural dimensions fall below 80 nanometers, the components are influenced by quantum effects in a disturbing manner, and they are dominated by quantum effects at dimensions of below about 30 nanometers. The increasing integration density of the components on a chip also leads to a dramatic increase in the waste heat.
One known possible successor technology to follow conventional semiconductor electronics is molecular electronics, in which both electron conduction and fundamental component functions are performed by suitable molecules. An overview of the field of molecular electronics is given by [1], for example. There are predominantly two types of molecules suitable for realizing electronic component functions. They are firstly molecules based on carbon nanotubes and secondly molecules based on polyphenylene.
Ellenbogen, J C, Love, J C (1999) “Architectures for molecular electronic computers; 1. Logic structures and an adder built from molecular electronic diodes”, MITRE Nanosystems Group, McLean, Va. discloses examples of molecular electronic molecules which, on account of their atomic structure, have the capability of fulfilling the function of an electrical conductor, an electrical insulator or a diode. Furthermore, the same document discloses molecules which perform the function of logic gates.
Accordingly, many component functions that are usually realized by conventional silicon microelectronics can also be realized by molecular electronic molecules, molecular electronics having the advantage of a considerably smaller dimension compared with conventional electronics. Whereas the smallest semiconductor structures that can be realized nowadays are of the order of magnitude of a few nanometers, structural dimensions down to the angstrom range are conceivable in the case of molecular electronic molecules. This means that significantly more compact arrangements of circuits are possible and the waste heat that occurs is a small amount in the case of molecular electronic circuits. Furthermore, Gittins, D I, Bethell, D, Schiffrin, D J, Nichols, R J (2000) “A nanometer-scale electronic switch consisting of a metal-cluster and redox-addressable groups” Nature 408:67–69 discloses that bispyridinium molecules have the property of occurring in two redox states the molecule having a greatly different electrical conductivity in the two redox states. Bispyridinium molecules have a low electrical conductivity in the oxidized state; the electrical conductivity is significantly higher in the reduced state. If a sufficiently high electrical voltage is applied to the molecule, then it is thereby possible to bring about the transition of the molecule from one state to the other. The redox transitions are reversible. On the basis of these properties, Gittins et al. describes the use of bispyridinium molecules as molecular electronic nanoswitches.
On account of their electrical properties and their small dimension, recently there has been much discussion concerning the use of organic, in particular biological, molecules for electronic applications. In this case, the problem arises that it is necessary to bind the molecular electronic molecules to suitable contacts, for example metallic conductors. This is necessary for the coupling to conventional silicon microelectronics.
Molecular electronic molecules are provided with functional groups which are suitable for chemical binding to a conductor material. The binding of thiol groups (SH), which occur at the end sections of many biomolecules, to conductor surfaces made of gold material has proved to be particularly suitable. The metallic conductors have to be provided in a suitable arrangement at a distance from one another which lies in the nanometers range, in order that the interspaces between two such conductor surfaces can be bridged by the molecular electronic molecules. The linear extents of molecular electronic molecules are typically of the order of magnitude of between a few angstroms and up to hundreds of nanometers.
The prior art discloses techniques for enabling a molecular electronic molecular to be coupled to conductor surfaces at two of its end sections. Thus, often a gold interconnect is fabricated by means of electron beam lithography, for example, said interconnect having a constriction of approximately 10 nanometers wide at its narrowest point. Such an interconnect is then torn away in a controlled manner at the narrowest point in a bending device, the distance between the two torn-away ends lying in the nanometers range and being adjustable by means of the bending mechanism. If a solution comprising molecular electronic molecules whose extent essentially corresponds to the distance between the two torn-away ends is then added, these molecules may bind for example via two thiol groups to the two torn-away ends, for example made of gold material, and electrically bridge the latter.
Even though this procedure is suitable for laboratory experiments, it is unsuitable for the integration of many molecular bridges. Applications of molecular electronics that are of economic interest aim, however, precisely to form a multiplicity of molecular electronic couplings between metallic electrodes. Therefore, the method described is too time-consuming and too expensive to be considered for the mass production of arrangements with a large multiplicity of molecular electronic components. Furthermore, the degree of reproducibility is low in the case of the described method for coupling metallic conductors by means of a molecular electronic molecule. It is difficult to carry out the tearing-away of the constricted gold interconnect by the bending device under controlled conditions. Furthermore, slightly different torn-away ends are obtained in each tearing-away operation, so that it is not possible to exactly set the distance between the torn-away ends. However, in order that molecular electronic molecules can immobilize on two conductor surfaces, a very precise matching of the linear extent of the molecule, on the one hand, and the distance between the conductor planes, on the other hand, is necessary.
As an alternative, there are proposals firstly to fabricate a contact electrode, then to apply the organic material (for example in the form of a monomolecular layer) and then to deposit the second electrode thereon. In this case, the process for fabricating the second electrode is to be chosen in such a way that the organic material is not damaged, which is a considerable restriction. Semiconductor technological method steps are often performed at high temperatures. Biomolecules are often unstable compounds which react extremely sensitively to excessively high temperatures. By way of example, it is known that many proteins denature irreversibly, and can thus be destroyed, at temperatures as low as slightly above room temperature. What is more, many semiconductor technological methods require the presence of aggressive chemicals (e.g. etching chemicals or deposition gases). Sensitive molecules can thereby easily be destroyed. By way of example, proteins and DNA strands can be decomposed if the pH value deviates too far from physiological pH values. For arrangements of economic interest, however, no consideration can be shown for the sensitivity of the molecules during the formation of the second electrode if the arrangements fabricated in accordance with the method are intended to be economically competitive. On the other hand, consideration must be shown for the intactness of the biomolecules, since biomolecules are frequently present only in low concentrations and are often expensive. Therefore, the method described is suitable only to a very limited extent for introducing sensitive, in particular temperature-sensitive, biomolecules.
To summarize, it can be stated that the known techniques for forming a coupling of two metal electrodes by means of a molecular electronic module have a series of disadvantages. Thus, structurally adequately defined arrangements cannot be fabricated in accordance with the known techniques and the methods are complicated, expensive and not suitable for economic mass production.
DE 100 13 013 A1 and U.S. Pat. No. 6,128,214 in each case describe a chemically synthesized electronic component comprising two crossing conductors between which an electrically addressable molecular species is arranged.
Furthermore, WO 01/27972 A2 describes a molecular electronic device which has two conductive contacts and also a conductive bridge connection between the two contacts. The conductive bridge connection can be brought, by means of a voltage pulse, to an arbitrary one of two predetermined states, to a first state of higher electrical conductivity or to a second state, in which the conductive bridge connection has a lower electrical conductivity by comparison with the first state. The conductive bridge connection has organic molecules.
U.S. Pat. No. 5,270,965 describes a method for driving a component having a pair of electrodes and, embedded between the latter, an organic insulation layer.
Other molecular electronic components are described in E. Emberly et. al, Principles for the design and operation of a molecular wire transistor, Journal of Applied Physics, 88 (2000) 9, pp. 5280–5282, 2000, J. Chen et. al., Large On-Off Ratios and Negative Differential Resistance in a Molecular Electronic Device, Science, 286, pp. 1550–1552, 1999, C. P. Collier et. al., Electronically Configurable Molecular-Based Logic Gates, Science, 285, pp. 391–394, 1999 and R. M. Metzger et. al., Unimolecular Electrical Rectification in Hexadecylquinolinium Tricyanoquinodimethanide, Journal Am. Chem. Soc., 119, pp. 10455–10466, 1997.